Transmissible spongiform encephalopathies (TSE) or prion diseases are a group of fatal neuro-degenerative disorders that affect both humans and animals. Most human TSE are sporadic, and about 10-15% of the cases are inherited as autosomal dominant traits. However, in several hundred cases, human TSE have been shown to be acquired by infection, which may be caused either by medical manipulations as in iatrogenic Creutzfeldt- Jakob Disease (CJD), or from the consumption of contaminated foods, as in kuru and variant CJD (vCJD). All prion diseases are believed to share the same pathogenic mechanism based on the conversion of the normal cellular prion (PrP c) into the infectious scrapie prion (prpSc). In animal TSE, such as scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle, it is believed that the PrP sc enters the host through the gastrointestinal tract, migrates to the spleen, and eventually causes disease in the CNS. However, in experimentally infected animals, PrP sc is first detected in the spleen even if the PrP Scis injected directly into the brain. The mechanism by which PrP sc moves in and out of the CNS is not known. Both PrP c and PrP sc are anchored to the membrane by glycosylphosphatidylinositol (GPI). Under some experimental conditions GPIanchored proteins can move from cell-to-cell. We hypothesize that inter-cellular transfer of PrP c or PrP sc facilitates the spread of infection. We developed a cell model to test whether PrP c is transfer from a human neuroblastoma cell line to a leukemia cell line, which lacks PrP c. We found that PrP c transfer requires cellular activation, cell-cell contact and is GPI anchor dependent. These findings strengthen the possibility that intercellular transfer of PrP c or PrP sc may play a role in the propagation of PrP L We propose: 1) to further characterize the intercellular transfer of prpC; 2) to investigate whether a similar transfer takes place for prpS; and 3) to explore whether intercellular transfer of PrP is important in the pathogenesis of prion disease in vivo using transgenic animals. The studies that we propose address very important and under studied aspects of prion diseases. The mechanisms that govern PrP sc propagation at the cellular level is one of the major remaining barriers to fully understanding the pathogenesis of prion diseases. New insights into this area will also lead to designs of more rational and effective treatment for prion diseases. [unreadable] [unreadable]